Anti-logarithmic or linear-exponential type converter circuits are used in a number of applications, such as automatic gain control systems where a signal, whose amplitude may vary within a wide range, is to be amplified such that a signal having a predetermined constant maximum amplitude can be output.
The simplest way of obtaining a linear-exponential conversion is to utilize the exponential characteristic of a bipolar junction of the PN type.
In this case, the converter circuits as designed by the skilled technician would be represented schematically by an operational amplifier having the cathode of a diode connected to its inverting input terminal, with the latter being connected to the output terminal through a resistor. The non-inverting input terminal of the amplifier is connected to a ground point. The anode of the diode is the input terminal.
For opposite polarity signals, the diode connection would be reversed.
The operational amplifier may be implemented to varying degrees of complexity using conventional techniques.
That circuit arrangement has, however, an inherent problem in that the value of the anode-cathode voltage of the diode is tied to such variations in the processing parameters as the "spread", and above all, temperature.